1. Field of Invention
This invention relates to cellular macromolecular organic structural material and particularly to the cellular synthetic rubber composition of a type having multiple uses such as carpet backing and to methods of preparing such cellular elastomer by the frothing of an aqueous latex and to frothing agents suitable for the modification of elastomer latex to permit the type of air entrainment involved in froth production.
2. Prior Art
Rubber trees produce an aqueous dispersion of elastomer known as natural latex. At an early date man learned that cellular products could be prepared by whipping air into an aqueous dispersion to prepare a froth which could be dried or otherwise cured into a cellular article.
Frothing agents modifying some of the surface tension relationships affecting the cell structure have been used for many decades in the frothing of latex. Substantially all surfactants have some effectiveness as frothing agents. Ordinary sodium stearate type of soap has been proposed for use as a latex frothing agent. The effectiveness of soap solutions in prolonging the stability of entrainment of air bubbles in aqueous systems is so well known as to be judicial knowledge.
In order to achieve commercially acceptable products, it has generally been necessary to employ a plurality of surfactants. Unfortunately a mixture of surfactants produces a great variety of complications, so that most blends of surfactants are commercially unsatisfactory as frothing agents for the aqueous dispersions of synthetic elastomers presently employed as latex. In some factories the secret know-how relating to the blending of surfactants for the formulation of an acceptable frothing agent was a trade secret which was carefully guarded.
Cellular cushioning is resiliently compressible because of the existence of gas cells throughout the flexible material. To the extent that the walls of each cell have elastic as distinguished from merely flexible characteristics, there is a greater bounce to the cellular plastic. Even a limited amount of elasticity improves the cushioning characteristics of the cellular cushion. It is feasible to have an elastomeric composition containing a significant amount of filler in the elastomer employed in a cellular cushion, even though such filler decreases the maximum stretch attainable by the loaded composition.
In recent years, many carpets have been made by a procedure in which threads are looped through a base to provide a pile fabric for the upperside of the carpet. At the underside of a carpet, the loops are anchored by embedding them in a cellular elastomeric cushion. The precursor for the cellular elastomeric cushion is deposited while the carpet, in an inverted position, is advancing through a coating zone. The precursor for the cellular elastomer cushion is generally a latex froth in which air is dispersed into a composition comprising an aqueous colloidal dispersion of an elastomer, a controlled amount of filler and an appropriate group of modifying agents for enhancing the curing and frothing of the cushion precursor.
In the manufacture of carpet backing using latex froth, it has been customary to utilize calcium carbonate as a filler. Some samples of calcium carbonate contain significant amounts of magnesium carbonate. Heretofore it has been found that frothing agents apparently functioning satisfactorily for the frothing of latex utilizing carbon as a filler were sometimes unsatisfactory when utilizing technical grades of calcium carbonate as fillers.
The tensile strength of the cushioning is of significantly greater importance in the backing for carpeting than for many other applications of cellular elastomeric materials. An important function of the backing is to achieve effective anchoring of the loops. The importance of adequate strength in the anchoring of the loops is about as great as the significance of the cushioning effect. The loading of the elastomer with finely divided calcium carbonate helps to achieve the high tensile strength which has characterized the cellular cushioning employed as carpet backing.
Many types of cellular plastics must be water-proof and vapor-proof and require effective closed cell structure. Carpet backing requires a very open cell structure such that gas can readily permeate from cell to cell. The water content of the latex must be evaporated from the backing while the carpet is passing through the curing oven. The permeation of the steam through the porous inter-connections between the cells is essential for the operability of the process of applying latex froth and curing it into a carpet backing.
Various blends of surfactants have been marketed as frothing agents. Some manufacturers of products, such as carpets, using latex froth have become accustomed to utilizing a selected brand of frothing agent. The market for frothing agents has not been destroyed by the widespread knowledge that a frothing agent was a controlled blend of surfactants. Notwithstanding the continuing research, prior art technologists could not satisfy many of the potential customers for a frothing agent.